How to decide whether your next dollar goes Roth or traditional

Every dollar you put toward retirement gets taxed once. You just get to pick when.

That's the entire argument. Traditional means you skip the tax now and pay it later, at whatever ordinary income rate applies when you withdraw. Roth means you pay today at your current rate and the money, plus everything it earns, comes out tax-free later if you follow the rules. The right answer is whichever rate is lower, and the honest problem is that you're comparing a rate you know against a rate nobody can promise you.

So you make a reasoned bet. And you hedge it.

The comparison almost everybody sets up wrong

People look at their top bracket today, say 24 percent, then assume they'll also be in the 24 percent bracket in retirement, shrug, and call it a wash. That's the wrong comparison.

The dollar you contribute today comes off the top of your income. It's saving you your highest marginal rate. But the dollar you withdraw in retirement doesn't come out of the top. It fills your tax return from the bottom up, starting with the standard deduction, then the lowest bracket, then the next one. If a traditional IRA withdrawal is a big share of your retirement income, a lot of it gets taxed at rates well below the rate you avoided.

That asymmetry is the structural case for traditional accounts during your peak earning years, and it's real.

The counterweight is everything that pushes your retirement income back up: a pension, Social Security for two people, rental income, required minimum distributions on a seven-figure pre-tax balance. Stack enough of those and your bottom brackets are already full before you touch the IRA. Then the withdrawal really does come out at the top.

A worked example, clearly labeled as an example

Illustrative only. Round numbers, no real person.

Say a 44-year-old engineer earning $180,000 puts $20,000 into his 401(k). At a 24 percent marginal rate, going traditional saves him $4,800 in federal tax this year. If that same money comes out three decades later in a year when his taxable income lands mostly in a 12 percent bracket, he's paid roughly $2,400 on it. He won the trade by about half.

Now change one fact. He has a $45,000 pension and his wife has one too. Add Social Security. By the time RMDs start, their bottom brackets are full before the IRA opens its eyes. Every dollar of that withdrawal is taxed at 22 or 24 percent. He broke even at best, and he spent thirty years pretending he was ahead.

Same contribution. Opposite verdict. The variable wasn't the market, it was what else was coming in the door.

Brackets, contribution limits and RMD ages move. Check the current numbers at IRS.gov rather than trusting anybody's article, including this one.

When traditional usually wins

• You're in your highest-earning decade and the deduction knocks you down a bracket.
• You live in a high income tax state now and you're seriously planning to retire somewhere that doesn't tax retirement income. That's a spread you can bank.
• Your pre-tax balance is modest and always will be, so the bottom-up fill works in your favor.
• Cash is tight and the deduction is the only way you can afford to contribute at all.

That last one matters more than it sounds. A traditional contribution you actually make beats a Roth contribution you skip.

When Roth usually wins

• You're early in your career and your rate is about as low as it's ever going to be.
• You're in a gap year. Sabbatical, business loss, the year you retired in March.
• You already have a large pre-tax pile and you're trying to keep future RMDs from becoming a tax problem.
• You're saving at the annual maximum. This one's subtle: the contribution limit is the same dollar figure either way, so $20,000 into a Roth shelters more real money than $20,000 into a traditional account, because the Roth dollars have already had the tax taken out. Maxing out in Roth is a quiet way to stuff more into the tent.

Don't overthink the match

If your employer matches, take the full match before you debate any of this. A 50 percent match is a 50 percent return on day one and no tax argument competes with it.

Under the SECURE 2.0 changes, employers are allowed to offer matching contributions as Roth. Many plans don't, and the match lands in a pre-tax bucket regardless of where your own money goes. Worth ten minutes on the phone with your plan administrator so you actually know which bucket your match is sitting in.

The reasons that have nothing to do with brackets

Four of them, and they get overlooked constantly.

Medicare surcharges. IRMAA raises your Part B and Part D premiums once your modified adjusted gross income crosses certain thresholds, based on a tax return from two years prior. Traditional withdrawals count toward that income. Qualified Roth withdrawals don't. Details and current thresholds live at Medicare.gov.

The survivor's tax bill. A married couple filing jointly gets roughly double the bracket width of a single filer. When one spouse dies, the survivor usually files single the following year with most of the same income still arriving. Same money, narrower brackets, bigger bite. Roth dollars protect the one who's left, and that's not a small thing to hand your wife.

What your kids inherit. Under current rules, most non-spouse heirs have to empty an inherited retirement account within ten years. If it's a traditional IRA, your son is pulling that money out during his own peak earning years, on top of his own salary. If it's a Roth, it comes out tax-free. Same balance, very different gift.

State income tax. Some states tax retirement income, some don't, some have carve-outs for pensions but not IRAs. If you're planning a move, this changes the math enough to be worth a conversation with a CPA who works in both states.

The window most people sleep through

Between the day you stop working and the day RMDs start, there's often a stretch of unusually low taxable income. Especially if you're delaying Social Security.

That's the cheapest time you'll ever get to move money from traditional to Roth. You control the amount, so you can convert just enough to fill up a lower bracket and stop before you spill into the next one, or before you trip an IRMAA threshold. Done a few years in a row, it can meaningfully shrink the pre-tax balance that RMDs will eventually attack.

It's also easy to overdo. Convert too much in one year and you've manufactured the exact tax problem you were trying to avoid. This is the part to run past a CPA with actual projections, not a spreadsheet you built at 11 p.m.

So where does the next dollar go

Match first. Then, if your marginal rate is high right now and you can name the lower-tax future you're aiming at, go traditional. If your rate is low this year, or your pre-tax balance is already the biggest thing on your net worth statement, go Roth.

And if you genuinely can't tell, split it. A 60/40 mix isn't indecision, it's a hedge against a tax code that's been rewritten six or seven times in your working life and will be again. Having both buckets means that in retirement you get to choose which one to draw from each year, and that dial is worth more than winning the argument.

None of this is advice for your situation. Before you convert anything, or change what your payroll deduction is doing, sit down with a fee-only fiduciary adviser and a CPA who can see your actual return.

The goal isn't to beat the IRS. It's to make sure the money you spent forty years earning doesn't arrive at your family's door with a bill stapled to it.